1. Field of the Invention
The present invention relates to a charged particle accelerating apparatus. More particularly, it relates to a charged particle accelerating apparatus for accelerating or storaging beams of charged particles such as electron beams so that radiation beams are generated from deflection parts of the apparatus.
2. Discussion of Background
FIG. 8 shows a conventional charged particle accelerating apparatus. In FIG. 8, a reference numeral 1 designates a storage ring for storing charged particles, a numeral 2 designates a beam line for introducing the charged particles into storage ring 1, numerals 3 designate deflecting electromagnets for forming a balanced circular orbit 4 by deflecting the charged particles, numerals 5 designate synchrotron orbital radiations (SOR) produced when the charged particles are deflected, the radiations being emitted outside to utilize them for, for instance, lithography, numerals 6 designate four-pole electromagnets for converging the charged particles, numerals 7 designate six-pole electromagnets for correcting a non-linear magnetic field of the deflecting electromagnets 3 or correcting chromaticity, a numeral 8 designates a high frequency cavity which compensates energy loss of the charged particles resulted from emission of the radiations, and accelerates the charged particles to have a given level of energy, a numeral 9 designates a kicker which shifts the balanced orbit 4 of the charged particles so that the particles can be easily injected in the storage ring 1, a numeral 10 designates a vacuum doughnut (or a vacuum chamber) for providing a path of the charged particles, a numeral 11 designates an inflector for permitting injection of the charged particles into the storage ring 1 from the beam line 2 and numeral 12 designates vacuum pumps for maintaining the vacuum doughnut to be in a highly vacuum condition.
The operation of the conventional apparatus will be described. The charged particles injected from the injecting beam line 2 into the storage ring 1 are deflected in the inflector 11 in a pulsating manner. The particles are circulated on an orbit slightly deflected from the balanced orbit 4 by the kicker 9, and after being circulated several times, they continue to circulate on the balanced orbit 4 (the balanced orbit is determined by the arrangement of the deflecting electromagnets 3 and the four-pole electromagnets 6). In the circulation of the charged particles, the high frequency cavity 8 accelerates the particles and the six-pole electromagnets 7 corrects unevenness of the magnetic fields in the radial directions of the deflecting electromagnets 3 or corrects the chromaticity. When the charged particles circulating along the balanced orbit 4 are deflected in the magnetic fields formed by the deflecting electromagnets 3, there take place electromagnetic radiations in the direction tangential to the orbit, the radiation being caused by Bremsstrahalung. The electromagnetic radiations are generated as radiation beams.
Generally, there are a number of radiation beam lines 5 and they increase efficiency in the discharged particle accelerating apparatus. In FIG. 8, a single radiation beam line 5 is shown for each of the deflecting electromagnets 3.
The vacuum chamber 10 is made of stainless steel having a high mechanical strength and facilitating baking. The interior of the vacuum chamber 10 is kept at a highly vacuumed condition by means of the vacuum pump 12 so that a shortened life of the charged particles due to energy loss by the collision of the particles to the molecules of a gas can be prevented. However, in the vacuum chamber of stainless steel surrounded by the deflecting electromagnet 3, a large amount of gas is generated from the stainless steel, whereby the vacuumed condition in the apparatus becomes poor. Thus, the gas generated in the vacuum chamber shortens the life of the charged particles.
There has been proposed a vaccum chamber made of an aluminum alloy. However, although the aluminum alloy vacuum chamber controls generation of gas by the synchrotoron radiation beams, it is impossible to carry out the baking at a high temperature because it has a low mechanical strength.
There has been used a vacuum chamber in a conventional electron storage ring as shown in FIG. 9. In FIG. 9, the same reference numerals as in FIG. 8 designate the same or corresponding parts. In the vacuum chamber made of stainless steel, SOR beams 5 are emitted from the balanced orbit 4. A numeral 24 designates a heat generating portion in the vacuum chamber, which is caused by radiation of the SOR beams.
In the apparatus as shown in FIG. 9, when the charged particles (electrons) are moved along the curved orbit by the deflectihg electromagnets 3, the SOR beams 5 are emitted in the direction tangential to the curved orbit. The intensity of the beams are very strong and have an extremely small diameter (less than 1 mm). Accordingly, when the beams strike the inner wall of the vacuum chamber 10 made of stainless steel, the surface of the inner wall is locally heated because thermal conductivity of the material is poor. Accordingly, as shown in FIG. 10, a thermal expansion takes plase in the inner wall portion of the vacuum chamber 10 in the direction indicated by an arrow mark 26. However, no thermal expansion takes place in the portion exposed in the atmosphere. Namely, only the portion facing the inside of the vacuum chamber expands. When deformed, the vacuum chamber 10 tends to form a recessed portion in the direction indicated by an arrow mark 27 to release the stress due to the expansion. However, it is difficult to form the recessed portion in the arrow mark direction 27 because the vacuum chamber has corner portions 28 in a form of an L (angle) shape. In such L-shape structure, it is difficult to cause the deformation of the corner portion inwardly. Accordingly, it is impossible to solely cause the deformation of that portion in the arrow mark direction 27. Namely, the vacuum chamber 10 is deformed to such extend that a force in the arrow mark direction 17 which is caused by the thermal expansion is balanced by a reactive force which is determined by the strength of the corner portion 8 and a material used for the vacuum chamber. Namely, the depth in the recessed portion is relatively small. Accordingly, the thermal expansion in the arrow mark direction 27 is more and less hindered and a large compression force is generated at the part of thermal expansion, i.e. the heat generating part 24. Further, the expanding part is heated at a high temperature (about 500.degree. C.) for a long time by the SOR beams, whereby there arises a problem of creaping. In addition, there is a problem of fatigue of the material because of a repeated stress. Further, there is a problem of production of the gas in the case that the vacuum chamber is made of stainless steel as described above.